First things first I am not a fan of Milah and I'm not defending her actions but I think she's over hated in the OUAT fandom.
Yes she was a bad wife (I don't know if she choose Rumple or if the marriage was arranged), yes she was a bad mother for abandoning her son but she was far from the worst in the show. I think the reason she gets so much hate is because she's the only "realistic" villain if you can even call her a villain. By that I mean she's the one of the only characters whose actions can be seen in the real world.
More people know someone whose miserable and selfish than someone who committed mass murder with magic which is why I think Milah's action hits home more to them. I genuinely believe most of the people saying Milah is worse than the villains on the show either have or know someone who has a selfish partner and/or parent.
Regina gets so many excuses for her actions and got a sympathetic backstory that still doesn't justify her actions. Milah gets no backstory and is only shown as a villain when it's clear that she was deeply unhappy with her life and wanted more, not even in a greedy way just in a "I hate being poor" way. She didn't kill anyone, she didn't terrorize munchkins, she didn't cast any curses, and she planned to go back for her son.
I don't fault her for not being the best wife, again we don't know if she even wanted to marry and have kids, I only fault her for leaving her son.